Modulation of incommensurately modulated structures studied by the maximum entropy method [Elektronische Ressource] / von Li Liang

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• Physik

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• Publié par: universitat_bayreuth
• Publié le: 01 janvier 2011
• Nombre de lectures: 20
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Modulation of Incommensurately
Modulated Structures Studied by
the Maximum Entropy Method
Von der Universit¨at Bayreuth
zur Erlangung der Grades eines
Doktors der Naturwissenschaften (Dr. rer. nat.)
genehmigte Abhandlung
von
Li Liang
aus Jinan, China
1. Gutachter: Prof. dr. Sander van Smaalen
2. Gutachter: PD. Dr. Natalia Dubrovinskaia
Tag der Einreichung: 26. 10. 2010
Tag der Kolloquiums: 17. 01. 20112，
34Contents
1 Introduction 1
2 Aperiodic crystallography and superspace 3
2.1Aperiodiccrystallography........................ 3
2.1.1 Incommensuratemodulatedstructures.. 4
2.1.2 Incommensuratecompositestructures... 6
2.1.3 Quasicrystals........................... 7
2.2Superspace........... 7
2.2.1 Reciprocalanddirectsuperspace..... 7
2.2.2 Symmetryinsuperspace.....................10
2.3Modulationfunctions................12
2.3.1 Differenttypesofmodulationfunctions..13
2.3.2 Modulationfunctionsusedinthepresentthesis........14
3 The Maximum Entropy Method 17
3.1ApplicationsoftheMEM ........................17
3.2PrincipleoftheMEM-BayMEM.........18
4 Integration of aperiodic crystals diffraction data 23
4.1 The EVAL15method...........................23
4.2Integratingofdiffractiondataofaperiodiccrystals...........23
4.3 Case study: data integration of Rb ZnCl by EVAL15.........25
2 4
4.3.1 Indexingofreflections......................29
4.3.2 Refinement.................30
iii CONTENTS
4.3.3 Finding the q-vector.......................35
4.3.4 Integration ........38
4.3.5 Dataanalysisanddatareduction.......45
4.4SpecialprobleminCCDdetectordataintegration............53
5 Incommensurately modulated Rb ZnCl 57
2 4
5.1Abstract..................................57
5.2Introduction.58
5.3Experimental61
5.3.1 Crystalgrowthandthediffractionexperiment.........61
5.3.2 Structurerefinements..............65
5.3.3 MEMcalculations.....68
5.4Discusion......................74
5.4.1 Natureofthemodulation...........74
5.4.2 Relationtothesolitonmodel.........79
5.4.3 Originofthemodulation..........81
5.5Conclusions.......................82
6 Incommensurately modulated Cr P O 85
2 2 7
6.1Abstract..................................85
6.2Introduction.86
6.3Themaximumentropymethod.8
6.4Experimental...............................93
6.4.1 Structurerefinements...93
6.4.2 MEMcalculations.....96
6.5Discusion.................................9
6.6Conclusions.106
7 Summary 109
8 Zusammenfassung 113
A Supplementary materials: Rb ZnCl 119
2 4CONTENTS iii
Declaration 149iv CONTENTSChapter 1
Introduction
Entropy is a concept used in thermodynamics to describe the state of order of a
system. This term is also used as a measure of amount of information in a data
set. The Maximum Entropy Method (MEM) is a general method for data analysis,
which is employed to extract the maximum amount of information from the data,
without the introduction of artifacts or assumptions concerning a model.
In crystallography, MEM has been used to reconstruct the electron density dis-
tribution in a unit cell allowed by the X-ray diffraction data. For aperiodic crys-
tals, many more parameters are needed to describe a structure, take the incom-
mensurately modulated structure as an example, which this thesis is focuses on,
basic-structure, atomic displacement parameters (ADPs) and an infinite number (in
principle) of parameters defining the modulation functions are used to describe one
independent atom in the unit cell. The conventional structure refinement methods
can determine a finite number of parameters at best, modulation functions are usu-
ally described by truncated Fourier series. Large number of parameters cannot be
refined due to interdependencies among them. Even some special shaped functions
(crenel function and sawtooth function) are used as modulation functions, but the
result of structure refinements is still restricted by the choice of parameters for the
modulation functions. The result may differ from the true functions and it may
not reflect the information content of the diffraction data. The MEM has been pro-
posed as a model-independent tool to obtain the most probable generalized electron
density in the unit cell of superspace. Analysis of this superspace electron density
12 CHAPTER 1. INTRODUCTION
map then provides a model-independent estimate of the modulation functions. This
thesis concentrates on the Maximum Entropy Method study of the modulated prop-
erties of incommensurate modulated structures. The modulation of the anharmonic
ADPs is found to be important, it affects the shape of mo functions and the
fitting of the model to the diffraction data.
The theory of aperiodic crystallography is described in Chapter 2. The basic
concept of incommensurate modulated structures, incommensurate composite struc-
tures and quasicrystals are given. The idea of superspace together with symmetry
options in superspace are introduced.
Theconceptofentropyispresented inChapter3. TheprinciplesoftheMaximum
Entropy Method and its applications in crystallography are described. BayMEM
(van Smaalen et al., 2003) and the Cambridge algorithm (Skilling and Bryan, 1984)
are introduced.
Chapter 4 focuses on the problem of extracting integrated intensities of Bragg
reflectionsfromareadetectordataforincommensuratelymodulatedcrystals. TheX-
ray diffraction data integration softwareEval15 is introduced. Integration of X-ray
diffraction data measured with CCD detector on beamline F1 (Hasylab, Hamburg)
is described step by step.
Chapter 5 reports on the application of MEM to the X-ray diffraction data of
incommensurately modulated rubidium tetrachlorozincate. The MEM study com-
bined with refinement method and difference-Fourier map study have uncovered the
modulated properties of both the harmonic and anharmonic atomic displacement
parameters of the atoms.
Chapter 6reportstheapplication ofMEMtotheX-raydiffractiondataofincom-
mensurately modulated Chromium pyrophosphate. The modulation functions, ac-
cording to atoms-in-molecules theory was presented. A new model was constructed
based on the results of the analysis of the MEM density. The modulated structure
in the disordered region was studied.